This disclosure relates in general to drilling wellbores through earth formations and, but not by way of limitation, to controlling gas in the wellbore fluid.
In deepwater drilling with a subsea blow-out preventer (BOP) there is risk of gas getting into the riser. Small amounts of gas may be undetected during the drilling process, particularly when drilling close to kick tolerance limits. Large expansion of gas in the drilling riser can occur to partially empty the riser. The volume of gas increases as it travels from the ocean floor toward the surface. Hydrostatic pressure can lead to riser collapse, the uncontrolled release of hydrocarbon at the surface when the diverter overloads or other problems.
In one embodiment of the invention, early kick detection is a consideration for rig safety and efficiency. The lower kick tolerances associated with deep water operations can be addressed by kick detection systems that are more sensitive and reliable that those which are usually available for conventional drilling operations. For example, lower fracture gradients than similar land or shallow water situations reduce the kick tolerance margin. However, kick detection in deepwater operations can be difficult. Two early warning signs of kicks are an increase in flow rate and pit volume. These signs are difficult to detect when drilling from floating vessels due to the nature of the drilling vessel motion. Waves can cause fluctuations in the pits that can complicate volume estimates. Similar problems affect the outflow rate measurement.
Failure to detect a gas influx lower in the wellbore in such an operation can lead to gas being circulated into a deepwater riser. This is even more likely when drilling with oil-based mud due to the solubility of the methane in the drilling fluid. Typically there is very limited pressure control at surface once the gas has been circulated past the BOP stack on the seafloor. The gas in the riser that is circulated during the drilling process can expand rapidly near surface and can lead to blow-out conditions. Furthermore, if the riser does become partially evacuated, there is also a risk of riser collapse.
When a kick is taken while drilling with a marine riser, there is a possibility that the gas can migrate or be circulated above the subsea BOP (SSBOP) stack. When this occurs, the choke and mud-gas separator are no longer available to control the flowrates when the riser gas reaches the surface. Even if the gas influx is detected early and the annular preventer is closed, some of gas influx may already be above the annular preventer. Additionally, there may be some gas above the annular preventer because detection of the kick did not occur until the gas had been circulated above the SSBOP stack.
An early flowcheck in the riser, immediately after shutting in the well, may show a flow indicating that the large bubbles are still rising. However, once all the small gas bubbles have been suspended in water based mud or dissolved in oil base mud, a flow check in the riser may falsely read negative even though there is gas in the riser. If a large amount of gas gets above the SSBOP stack, it can rise rapidly and carry a large volume of mud out of the riser at high rates. One way of managing gas in the riser is to avoid such situations.